CO2 terminated rubber for plastics

ABSTRACT

A process for the manufacture of a baleable lithium carboxylate polymer. The process includes polymerizing at least one conjudated diene in the presence of an organolithium initiator to a substantial completion. Thereafter, the polymerization process is terminated by the addition of carbon dioxide.

FIELD OF INVENTION

The present invention relates to polymerization of monomers and to the use of the resultant polymers as additives in other polymeric systems. More particularly, this invention relates to a process of polymerizing vinyl compounds and, more specifically, a method of terminating the polymerization process to produce a compound having high bulk viscosity, but low solution viscosity.

BACKGROUND OF THE INVENTION

Certain engineered plastics, such as styrene-maleic anhydride copolymers (SMA's) and high impact polystyrenes (HIPS), are prepared in the presence of a rubber, for example, a polybutadiene or a styrene-butadiene copolymer, to enhance toughness, impact strength and other properties. A low solution viscosity additive rubber can facilitate the dispersion of the rubber in the plastic phase. Moreover, early in polymerization of HIPS, phase separation begins because of the immiscibility of the rubber within the polystyrene being formed and depletion of the styrene phase. Furthermore, in SMAs, a low solution viscosity may improve the clarity and the gloss of the resultant product.

While low solution viscosity of the additive rubber is highly desirable, it makes commercial handling difficult. Moreover, low solution viscosity typically leads to a liquid or semi-liquid material that is difficult to package and ship. Accordingly, a relatively high bulk viscosity material capable of being baled into a shippable and easy to handle form is desirable.

Significant work has been conducted in the art of polymerizing vinyl compounds, particularly conjugated dienes. However, there has been no suggestion to use the present inventive polymerization technique for termination of the reaction to achieve a high bulk viscosity, low solution viscosity rubber compound.

The present invention uses CO₂ termination of the polymerization process to yield a highly desirable product. Importantly, while the use of CO₂ termination of polymerization has been utilized before, CO₂ reaction termination has previously been performed to provide a reactive product by the immediate protonation or other activation of the polymeric material. For example, U.S. Pat. No. 3,070,579 teaches reacting a living polymer, i.e., a polymerizable chemo-aromatic hydrocarbon having reactive negatively charged end groups, with a compound such as a CO₂, CS₂, 1,2-propylene oxide or ethylene oxide while having the reactants dissolve in a liquid. The patent further states that because of the reactive end groups, the bifunctional polymeric product is reacted with other groups or compounds to form many compositions.

The present invention advantageously provides a low molecular weight carboxylated polymeric material having a bulk viscosity particularly suitable for addition to SMA or HIPS resins.

SUMMARY OF THE INVENTION

The present invention provides a method of forming a low solution viscosity, lithium carboxylate polymer including monomer units of vinyl compounds. Generally, the process includes polymerizing at least one conjugated diene in the presence of an organolithium initiator substantially to completion and terminating the reaction via the addition of CO₂. The combination of steps in the present process provides terminating living anionic polymers with CO₂, leaving the polymer chain end as a lithium carboxylate (P—COO⁻Li⁺). In contrast to prior CO₂ termination processes, the present invention does not protonate nor further react the resultant polymer. Rather, the present invention advantageously seizes on the advantages of using a high bulk viscosity, low solution viscosity rubber as demonstrated by the carboxylate form of the material. The resultant product is a rubbery material with high bulk viscosity e.g. Mooney Viscosity above about 45, preferably above about 60, and good resistance to cold flow. Solutions of the rubbery material, in monomers used to form plastic resin, are generally low in solution viscosity, e.g. below about 75 cP, preferably below about 50 cP, most preferably below about 45 cP. The low solution viscosity provides higher gloss and allows greater rubber content to equivalent power levels.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention is directed to the development of low solution viscosity, high bulk viscosity polymer systems. These polymers are preferably homopolymers of conjugated dienes terminated with carboxylate groups. To form these polymers, conjugated dienes are polymerized in the presence of an organometallic initiator in a solvent.

The polymerization is carried out by living anionic polymerization, and the polymerization is terminated by the addition of CO₂ groups. Living polymerizations are polymerizations in which propagating centers do not undergo either termination or transfer. After 100% conversion is reached, additional polymerization takes place by adding more monomer to the reaction system. The added monomer is also polymerized quantitatively. Such polymerizations offer the potential for producing structures with defined end groups and block copolymers.

Although conjugated diene homopolymers are preferred products, conjugated diene copolymers may also be highly desirable where the comonomers impart desirable properties and do not detract from the polymer properties. The comonomers may be vinyl arenes, including vinyl aromatic hydrocarbons having alkyl, aralkyl, or cycloalkyl groups attached to the aromatic nucleus and preferably having no more than 20 carbon atoms. Typical of these aromatic comonomers are styrene, α-methyl styrene, vinyl toluene, ethyl styrene, p-cyclohexyl styrene, vinyl naphthalene, vinyl ethyl naphthalene, vinyl methyl naphthalene, vinyl butyl naphthalene, vinyl diphenyl, vinyl diphenylethane, 4-vinyl-4′-methyldiphenyl, and the like. Preferably, such comonomers have no more than 20 carbon atoms. Where such comonomers are to be used, generally at least 1%, preferably at least 5% by weight should be used and as much as 60%, preferably no more than 30% of the comonomer may be used.

A high vinyl content in the carboxylate terminated conjugated diene homo- or co-polymers might be desired. Suitable 1,2-vinyl modifiers may be added to the polymerization mixture to increase the vinyl content to as high as 90% of the conjugated diene monomer units. Exemplary 1,2-vinyl modifiers include one or more of hexamethylphosphoric acid triamide, N,N,N′,N′-tetramethylethylene diamine, ethylene glycol dimethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, tetrahydrofuran, 1,4-diazabicyclo [2.2.2] octane, diethyl ether, triethylamine, tri-n-butylamine, tri-n-butylphosphine, p-dioxane, 1,2-dimethoxy ethane, dimethyl ether, methyl ethyl ether, ethyl propyl ether, di-n-propyl ether, di-n-octyl ether, anisole, dibenzyl ether, diphenyl ether, dimethylethylamine, bis-oxalanyl propane, tri-n-propyl amine, trimethyl amine, triethyl amine, N,N-dimethyl aniline, N-ethylpiperidine, N-methyl-N-ethyl aniline, and N-methylmorpholine.

One effective catalyst system for living anionic polymerizations is a hydrocarbyl lithium. Hydrocarbyl lithium compounds may be represented by the formula R¹Li wherein R¹ is a C₁-C₂₀ hydrocarbyl radical, advantageously a C₁-C₂₀ aliphatic radical, preferably, C₃-C₆ but may also be C₆-C₂₀ cycloaliphatic or aromatic, preferaby C₆-C₁₂. Preferred R¹Li compounds are n-butyl and sec-butyl lithium. Other suitable R¹Li compounds include but are not restricted to those in which the R¹ groups are ethyl, n-propyl, isopropyl, n-amyl, sec-amyl, sec-hexyl, n-hexyl, octyl, nonyl, decyl, dodecyl, octadecyl, phenyl, tolyl, dimethyl-phenyl, ethylphenyl, naphthyl, cyclohexyl, methylcyclohexyl, ethylcyclohexyl, cycloheptyl, allyl, 2-butenyl, 2-methyl butenyl, cyclopentylmethyl, methylcyclopentylethyl, phenylethyl, cyclopentadieneyl, naphthyl, phenylcyclohexyl, etc. Generally, the catalyst is used in a proportion of about 0.15-20 mmol of initiator per 100 g of monomer.

The polymerization is advantageously conducted in a solvent. Hexane is preferred as well as other C₃-C₂₀ alkanes, preferably C₅-C₁₀ alkanes, such as butane, pentane, heptane, octane, nonane, decane, etc. However, other nonaromatic hydrocarbon solvents such as cycloalkanes, e.g., cyclohexane, methylcyclohexane, cycloheptane, etc., may also be used. C₆-C₂₀ cycloalkanes are preferred, more preferably C₅-C₁₀. Toluene and other aromatics may act as telomerizing agents and thereby reduce the average molecular weight of the product. However, where this is not critical, aromatic solvents may be used. Advantageously, a butadiene concentration of about 15-50% is desirable, preferably about 20-25%.

In the catalyst system and polymerization process, the standard precautions against contamination of an organometallic system with impurities such as water, air, etc., which deactivate or reduce the efficiency of the system should be taken. Consequently, the solvent, reagents, reactor and atmosphere in the reactor area are treated accordingly to avoid such contaminants. Advantageously, less than 25 ppm, preferably less than 5 ppm, of water is present during polymerization.

The polymerizations can be conducted in autoclaves, pressurized reactors or bottles capable of withstanding the pressures generated at the temperature used. Preferably, the pressures will be in a range of about 34-760 kPa, more preferably between about 200 and 700 kPa. Temperatures are preferably between about room temperature to about 120° C., more preferably between about 30 and 100° C.

While a substantial amount of polymerization is effected within one hour, additional polymerization can be effected at longer residence times, e.g. three hours. However, generally 6 hours or more are desired for greater yields, and while polymerization is generally substantially completed within 16 hours, depending on the temperature, there is no harm or adverse result in allowing polymerization to continue 70 hours or more.

When polymerization is completed, the catalyst is deactivated by the addition of a small amount of CO₂, which is preferably added in an amount of at least about ½, up to more than 1, molar equivalent of lithium initiator. The CO₂ terminates the living polymer chains, resulting in carboxlyate end groups on most of the polymer chains. It is added to the polymerization mixture by bubbling through in a gaseous form. The CO₂ then reacts with the reactive end groups of the living polymer chains to effectively end the living polymerization. The resulting polymer has carboxylate end groups which are stabilized by the Li⁺ initiator residues present in the polymerization mixture along with some undesired covalently coupled polymer chains.

A small amount of antioxidant, such as di-t-butyl cresol is preferably added to the polymer product. The antioxidant is preferably added in an amount less than 4 weight percent, more preferably less than 2 wt. % of the total solution. The polymer can be recovered and dried before processing, preferably by drum drying at an appropriate temperature for evaporation of remaining solvent. Alternatively, a steam desolventization method is used to recover the polymer product.

The number average molecular weight (M_(n)) of the carboxylate terminated polymer product is advantageously in the range of about 100,000 to 300,000, preferably about 150,000 to 250,000. Furthermore, narrow molecular weight ranges may be desired for specific properties. Molecular weights reported herein are determined by Dilute Solution Viscosity (DSV).

Other polar additives, such as maleic anhydride can be added to decrease solution viscosity if so desired. In addition, additives such as tetramethyl ethylene diamine, 2-ethylhexanoic acid, acetonitrile, and mixtures thereof may be added to the rubbery polymer composition to further improve the solution viscosity. The precipitated product can be filtered and washed with more alcohol and finished by addition of suitable stabilizers and inhibitors followed by drying according to methods known in the polymer field. If desired, the product may be extracted with acids, bases, complexing agents, etc., to reduce catalyst residues to a low level prior to the addition of stabilizers or inhibitors.

After formation, the rubbery polymer is then advantageously mixed with other polymer systems such as HIPS, SMAs, or acrylonitrile styrene butadiene copolymers (ABS). These polymer systems are reinforced by the addition of rubbers such as those of the present invention.

EXAMPLES Example 1 Increased Bulk Viscosities Via CO₂ Termination

Mooney viscosities (bulk viscosity measurement) were determined for different living polymers which were split in two and terminated with a proton source (H⁺ from water or isopropanol) or CO₂:

As can be seen in Table 1, CO₂ termination, followed by antioxidant additives and drying showed significantly increased bulk viscosity. TABLE 1 Mooney Viscosities Mooney Mooney Viscosity Viscosity with M_(w) with H⁺ Termination CO₂ Termination Polybutadiene 151 K 23.7 112.2 Polybutadiene 163 K 35.8 94.0 Butadiene/styrene 160 K 27.5 60.5 copolymer Butadiene/styrene  18 K 40.5 75.9 copolymer

In each of these examples, the CO₂ was added to the polymerization mixture to terminate the polymerization. Following the CO₂ addition, an antioxidant was added to precipitate the polymer product and the product was drum dried.

EXAMPLE 2 Solution Viscositites of P—CO₂ Li with Additives

A living polybutadienyl lithium was treated with CO₂ and this P—CO₂ ⁻Li⁺ was dissolved in toluene for solution viscosity determinations (in toluene). Various additives were also added to further reduce solution viscosity. Solution Viscosity (cP) Additive (amount)* 92.8 none 26.8 maleic anhydride (10%) 27.1 maleic anhydride (5%) 27.5 maleic anhydride (1%) 89.4 none 93.5 none 92.0 isopropanol (1.5%) 67.9 tetramethyl ethylene diamine 29.5 2-ethylhexanoic acid (1.5%) 92.5 acetonitrile (1.5%) *% based on weight of polymer

As seen in the above examples, additives such as maleic anhydride or carboxylic acids like 2-ethylhexanoic acid work cooperatively with the CO₂ terminated polybutadiene to significantly decrease solution viscosities. Similar results were observed for solutions of these rubbery polymers in a monomer such as styrene.

Example 3 CO₂ Terminated-Continuously Polymerized Polybutadiene

A living lithium high vinyl polybutadiene was treated with CO₂ with the following results: Mooney if H⁺ terminated  13 Mooney if CO₂ terminated  46 Solution Viscosity after CO₂ 119 cP Solution Viscosity after CO₂  72 cP treatment and addition of maleic anhydride Comparative Example: H⁺ terminated polybutadiene (Mooney Viscosity)  35 H⁺ terminated polybutadiene (solution viscosity)  97 cP

The H⁺ terminated polybutadiene used in the comparative examples was a medium-vinyl, unmodified polybutadiene with a M_(w) of about 200,000, and was obtained from Firestone Polymers, Inc.

Various modifications and alterations that do not depart from the scope and spirit of this invention will become apparent to those skilled in the art. This invention is not to be unduly limited to the illustrative embodiments set forth herein. 

1-10. (canceled)
 11. A baled, low solution viscosity polymer comprising conjugated diene contributed monomer units and carboxylate terminal groups, wherein said polybutadiene comprises a solution viscosity below about 75 cP and a Mooney Viscosity above about
 45. 12. The polymer of claim 11 further comprising monomer contributed units selected from the group comprising vinyl aromatic hydrocarbons.
 13. The polymer of claim 11 further comprising an initiator residue such as Li⁺.
 14. The polymer of claim 12 wherein said vinyl aromatic hydrocarbons are one or more of styrene, α-methylstyrene, vinyl toluene, ethyl styrene, vinyl naphthalene, vinyl ethylnapthalene, vinyl methyl naphthalene, vinyl butyl naphthalene, vinyl diphenyl, vinyl diphenylethane, 4-vinyl-4′-methyldiphenyl, and the like.
 15. The polymer of claim 11 wherein the polymerization was initiated by a hydrocarbyl lithium.
 16. The polymer of claim 15 wherein said hydrocarbyl lithium is n-butyl lithium.
 17. The polymer of claim 11 wherein said solution viscosity is below about 75cP.
 18. The polymer of claim 11 wherein the Mooney Viscosity is above about
 45. 19. A rubber-modified high impact polystyrene or styrene-maleic anhydride polymer wherein said rubber comprises a baleable carboxylate terminated polybutadiene of claim
 11. 20. (canceled)
 21. The polymer of claim 11 having a number average molecular weight from about 100,000 to 300,000.
 22. The polymer of claim 21 having a number average molecular weight from about 150,000 to 250,000. 